1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display apparatus.
2. Description of the Related Art
An active matrix type liquid crystal display (LCD) apparatus which is provided with thin film transistors (TFT's) formed with an amorphous silicon (a--Si) film as switching devices has gained public attention. By constructing a TFT array with the a--Si film which can be formed on an inexpensive glass substrate at a low temperature, a panel display (a flat type television) which features a large area, a high definition, a high picture quality, and a low cost may be provided.
FIG. 1 is an equivalent circuit of a picture element of a substrate for a liquid crystal driving semiconductor device used for such a liquid crystal apparatus. As shown in the figure, in switching a pixel by an address pulse, the potential of a picture element electrode drops, by coupling through a floating capacitance (C.sub.gs) between the gate and the source of the TFT. To prevent the potential drop of the picture element, a storage capacitance (C.sub.s) is disposed in parallel with a liquid crystal layer (LC). In FIG. 1, reference numeral 1 is an address line and reference numeral 2 is a data line.
The sectional view of the substrate for the liquid crystal driving semiconductor device array which constitutes the above mentioned picture element circuit is shown in FIG. 2, for example. In other words, an address line 1; a gate electrode line 1a which is connected to the address line 1; a storage capacitance line 1b, which are connected to the address line 1; an insulator film 4, which covers the address line 1, the gate electrode line 1a, and the storage capacitance line 1b; a TFT semiconductor thin film 5, which is formed on the insulator film 4 over the gate electrode line 1a; a drain electrode 2a and a source electrode 2b, which are formed on both the ends of the TFT semiconductor thin film 5; a display electrode 6, which is formed on the insulator film 4 over the storage capacitance line 1b for providing a storage capacitance between the storage capacitance line 1b and the display electrode 6; and a data line 2, which is formed nearly perpendicularly to the address line 1 on the insulator film 4 are disposed on one surface of a glass substrate 3. The TFT drain electrode 2a is connected to the data line 2. The source electrode 2b is connected to the display electrode 6.
In the above mentioned construction, light does not penetrate into the storage capacitance line 1b. Thus, the aperture ratio is adversely decreased by the area of storage capacitance line 1b. Consequently, a requirement for decreasing the area of the storage capacitance line 1b as small as possible arises.
Moreover, in the liquid crystal driving semiconductor device substrate, a distortion of the wiring pattern may cause a shortcircuit between the data line 2 and the display electrode 6 and thereby a point defect takes place. Thus, as shown by a sectional view of FIG. 3, the display electrode 6 and the insulator film 4 which is formed under the display electrode 6 (hereinafter the insulator film 4 is named the first insulator film) are coated with a second insulator film 7 except for the portion for connecting the display electrode 6 and the source electrode 2b. The second insulator film 7 prevents such a point defect from taking place.
However, in the above mentioned construction, since the inter-layer insulator film is formed with two layers of the first insulator film 4 and the second insulator film 7, the following drawbacks result. Normally, the second insulator film 7 is formed by a CVD method or plasma CVD method. Since such equipment is expensive, two depositions of CVD film will increase the production cost. In addition, to properly set the thickness of the insulator film between the gate electrode line 1a and the semiconductor thin film 5, it is necessary to decrease the thickness of the first insulator film 4. Thus, a shortcircuit tends to take place between the display electrode 6 and the storage capacitance line 1b.
In the above construction of the substrate for liquid crystal driving semiconductor device the storage capacitance Cs can be expressed by the following equation. EQU C.sub.s =.epsilon..sub.0 .times..epsilon..sub.s .times.S/d
Wherein .epsilon..sub.s is the dielectric constant of the insulator film, d being the film thickness, S being the area of the electrode, and .epsilon..sub.0 being the dielectric constant of a vacuum.
To obtain a large capacitance, it is preferable to increase the dielectric constant of the insulator film, .epsilon..sub.s, and the area of the electrode, S, and to decrease the film thickness of the insulator film, d. However, the dielectric constant of the insulator film, .epsilon..sub.s, depends on the material. For conventionally used films, in the case of SiO.sub.x, the dielectric constant is 4. In the case of SiN.sub.x, it is 7. In other words, it is difficult to obtain a large value of the dielectric constants when SiO.sub.x and SiN.sub.x, are used. Moreover, the film thickness, d, cannot be remarkably decreased due to restrictions of the breakdown voltage, leak current, and the pin hole density. The area of the electrode, S, should be decreased so as to improve the aperture ratio. Thus, an insulator film with a high dielectric constant is required. In addition, the increase of the aperture ratio is further required with decreasing the size of picture elements. When data is written, a data voltage drop by the following equation takes place. EQU .DELTA.V.sub.d =C.sub.gs .times.V.sub.s (C.sub.LC +C.sub.S +C.sub.gs)
(where .DELTA.V.sub.d is a voltage drop due to an address pulse switching; C.sub.gs is a floating capacitance between the gate and the source; C.sub.LC is a capacitance of the liquid crystal layer; C.sub.s is the value of storage capacitance; and V.sub.g is a gate pulse voltage.) To decrease the voltage drop, the value of the storage capacitance C.sub.s should be large. Since .DELTA.V.sub.d causes a DC component in the liquid crystal, .DELTA.V.sub.d should be 1% or less of the data voltage, V.sub.d. In the conventional LCD, since C.sub.gs, is around 0.1 pF and C.sub.LC is around 0.2 pF, C.sub.s should be 100 C.sub.gs, that is, around 10 pF. In addition, as the data holding time (.tau.=C.sub.s R), is required 5 times of the frame time, namely 5.times.33 ms, to reduce flickers. Thus, normally, C.sub.s R should be greater than or equal to 5.times.33 ms.
On the other hand, since C.sub.S R=(.epsilon..sub.0 .times..epsilon..sub.s /d).times.S.times.(d/S).times..rho.=.epsilon..sub.0 .times..epsilon..sub.s .times..rho., then EQU .epsilon..sub.s .times.p.gtoreq.1.9.times.10.sup.12
Thus, in the case of SiO.sub.2 (.epsilon..sub.s =4), .rho..gtoreq.4.7.times.10.sup.11 .OMEGA.cm; in the case of SiN (.epsilon..sub.s =7), .rho..gtoreq.2.7.times.10.sup.11 .OMEGA.cm; and in the case of TaO (.epsilon..sub.s =30), .rho..gtoreq.6.2.times.10.sup.10 .OMEGA.cm. However, actually, R is a parallel resistance of R.sub.cs and the off resistance of the TFT, R.sub.off. Thus, the resistance should be twice the above mentioned value. For example, when TaO is utilized, .rho..gtoreq.1.2.times.10.sup.11 .OMEGA.cm. However, in the case of TaO, since .rho. is in the range from 3.times.10.sup.9 .OMEGA.cm to 5.times.10.sup.10 .OMEGA.cm, the above condition is not satisfied. In other words, to increase the value of the storage capacitance, C.sub.s, it is necessary to use a material with a large value of the dielectric constant .epsilon..sub.s and a large value of the resistivity .rho..
To prevent a point defect due to a distortion of the wiring pattern, a substrate for liquid crystal driving semiconductor device where the display electrode 6 is coated with an insulator film has been studied. However, in such a construction, two layers as the inter-layer insulator film should be deposited by means of the CVD method or the plasma CVD method which requires expensive equipment. In addition, a shortcircuit tends to take place between the storage capacitance line 1b and the display electrode 6. On the other hand, there is a requirement of decreasing the area of the storage capacitance line 1b as small as possible so as to prevent the aperture ratio from decreasing.
Therefore, an object of the present invention is to provide a liquid crystal display apparatus with a storage capacitance line having a small area so as to improve the aperture ratio.
Another object of the present invention is to provide a liquid crystal display apparatus for clearly displaying a picture with small picture elements for accomplishing a high definition display.
Another object of the present invention is to provide a liquid crystal display apparatus for completely preventing a shortcircuit between a storage capacitance line and a display electrode.
Another object of the present invention is to provide a liquid crystal display apparatus which can be produced with a high cost performance.